Update usage info

trigger_copr

     warn ""
     warn "Options:"
     warn ""
+    warn "  -b BRN  build from branch BRN (default: last tag)"
     warn "  -n      dry mode, don't do anything (just show"
     warn "          what would be done)"
     warn "  -r REL  use REL as Release number in SPEC file"
     warn "  -u URL  use URL as base (to get last tag and"
     warn "          compose Source0 in SPEC file)"
     warn ""
-    warn "If -r or -v are not specified, git-describe master"
-    warn "is consulted."
+    warn "If -b is not used, build is launched from last tag available"
+    warn "in the GitHub repo.  In that case, Release is '1' and Version"
+    warn "is deduced from the tag by removing the initial 'v'."
+    warn ""
+    warn "If -b is used, the project repo is temporarily clonded, and"
+    warn "both Version and Release are found by consulting git-describe"
+    warn "on the specified branch."
     exit 2
 }